1. Field of the Invention
The present invention relates to a radiation image processing apparatus and processing method thereof that capture a radiation image by detecting and processing radiation with a radiation detector.
2. Description of the Related Art
In the medical field, for example, a radiation image processing apparatus is extensively used, which exposes a subject (patient) to radiation emitted from a radiation source, detects and processes the radiation that has passed through the subject with a radiation detector to obtain radiation image information. The obtained radiation image information is then displayed on a display unit for diagnostic use or the like.
FIG. 7 schematically illustrates the configuration of a typical radiation detector 2. The radiation detector 2 includes a plurality of detecting elements 11-55 arranged in a matrix form.
The detecting elements 11-55 are connected to gate lines 4a-4e extending from a gate driving circuit 3, and also to signal lines 6a-6e extending from a signal reading circuit 5. A reading control circuit 7 provides control signals to the gate driving circuit 3 and the signal reading circuit 5, to select one of the gate lines 4a-4e and one of the signal lines 6a-6e, respectively. Thus, the charge information of the specified one of the detecting elements 11-55 can be read out.
FIG. 8 illustrates an equivalent circuit of one of the detecting elements 11-55. Each of the detecting elements 11-55 includes a common electrode 8 supplied with a bias voltage from a power source B, a converting layer 9 such as an amorphous selenium (a-Se) layer given by vapor deposition, for converting radiation X to a charge signal, a pixel electrode 10 for collecting the charge generated by the converting layer 9, a storage capacitor C for holding the collected charge, and a transistor switch Tr for providing the charge information held in the storage capacitor C to an external circuit. The source, gate, and drain terminals of the transistor switch Tr are connected to the storage capacitor C, one of the gate lines 4a-4e, and one of the signal lines 6a-6e, respectively.
Exposure of the radiation detector 2 to excessive radiation X with the transistor switch Tr in OFF state causes a large amount of charge to be stored into the storage capacitor C, resulting in an excessive increase of the drain-source voltage, which may damage the transistor switch Tr.
To protect the transistor switch Tr from the damage due to high voltages, the prior art disclosed in Japanese Laid-Open Patent Publication 2000-075039 has the power source B apply a negative bias voltage to the common electrode 8 so that the polarity of the common electrode 8 is set to be the same as that of the gate terminal of the transistor switch Tr in the OFF state. In this case, application of radiation X causes the potential Vs of the storage capacitor C to drop toward a negative potential. When the potential Vs drops below the negative potential Vg of the gate, the transistor switch Tr is forced to turn on, allowing discharge of the storage capacitor C and thereby preventing damage to the transistor switch Tr.
In the radiation detector 2 of the above configuration, exposure of the radiation detector 2 to excessive radiation X forces the transistor switch Tr to turn on, allowing the detecting elements 11-55 to release charge, which will be referred to hereinafter as leak charge. Therefore, if the period between completing irradiation with the radiation X and starting to read out the charge information from each detecting element 11-55 with the signal reading circuit 5 is short, acquired radiation images 60 may include inappropriate artifacts 62a and 62b generated by the leak charge, as shown in FIG. 9.
If the charge information is read out from the detecting elements 11-55 in sequence from the gate line 4a to gate line 4e, that is, in the direction indicated by the arrow shown in FIG. 9, the charge information read out from the detecting elements 11-15 additionally includes the leak charge of the detecting elements 11-55 and thus is greatly affected by the leak charge. The charge information read out from the detecting elements 51-55 of the gate line 4e, on the other hand, is less affected by the leak charge since the leak charge of the detecting elements 11-45 have already been read out.
The contrast of the radiation image 60 is typically controlled so that a suitable brightness is obtained at the center of the subject image 64 (which is the image of a head in FIG. 9). In the case shown in FIG. 9, the radiation image has a lower portion of a central section of the subject image 64 (the area between the dashed-dotted lines 66a and 66b), where the neck is located. If, however, the contrast of the center of the radiation image 60 is controlled so that the neck portion of the subject image 64 is unaffected by the leak charge from the detecting elements 11-55 corresponding to the neck portion, artifacts 62a and 62b are generated on the subject image 64 due to the leak charge of the detecting elements 11-55 located outside the dashed-dotted lines 66a and 66b. 